1. Technical Field
This invention relates to the technology of making low-cost and durable three-way platinum group metal catalysts, and more particularly to techniques for improving the dispersion of the precious metal catalyst deposited from an aqueous phase.
2. Discussion of the Prior Art
Most commercial applications of precious metals as catalysts use a procedure that coats a substrate with an aqueous solution containing precious metal salt solutions such as chloroplatinic acid to deposit platinum, or rhodium nitrate to deposit rhodium, or palladium nitrate to deposit palladium. The coated substrate is dried at about 393.degree. K and then calcined from 650.degree.-900.degree. K to decompose the salts to their respective metal and/or metal oxide particles. The dispersion distribution for each metal resulting from this procedure can be quantitatively characterized as a benchmark, as measured from the activity of these catalysts in structure sensitive reactions such as oxidation, reduction, and hydrogenolysis.
Techniques which have been used experimentally to vary the dispersion of precious metal catalysts have included (i) use of a different pretreatment procedure, (ii) use of composite oxide, and (iii) use of solution additives which act directly on the metal ion to effect its precipitation or distribution during impregnation.
In the use of a different pretreatment procedure, the deposited precious metal compound is treated at high temperatures to either sinter or redisperse the catalyst. As disclosed in the article by H. C. Yao et al, "Surface Interactions in the Pt/.sub..gamma. -Al.sub.2 O.sub.3 System", Journal of Catalysis, 59 (1979) 365-374, treatment may be carried out under oxidizing or reducing environments. It has been shown in the case of platinum catalysts that the platinum may be dispersed under an oxidizing environment and sintered under a reducing environment. The disadvantage of such a pretreatment procedure is that it is severe and sinters the support, causing catalyst deactivation.
In using a composite oxide, the properties of the catalyst support (typically Al.sub.2 O.sub.3 ) are modified by depositing additives in the form of a base metal and rare earth oxide. As shown in the article by H. C. Yao et al, "Interactions of Base and Noble Metals with Insulator Supports", Metal-Support and Metal Additive Effects in Catalysis, (B. Imelik et al, Eds) (1982) 159-169 dispersion of noble metal on such composite oxide is different from that obtained on pure alumina. The use of the base/rare earth metal oxide could adversely alter the activity of the catalyst.
With respect to using additives to change the solution phase chemistry of the metal ion (as discussed in the article by Y.-J. R. Huang et al, "The Effect of Solution Variables on Metal Weight Loading During Catalyst Preparation", Applied Catalysis, (1986) 241-248 salts, such as nitric acid and ammonium hydroxide, have been used to impregnate the metal ion. Such salts affect metal ion precipitation directly; they also are relatively expensive and very cumbersome in use.
It would be desirable if a method could be devised using a solution phase chemistry additive which acts indirectly to improve the dispersion of the platinum group catalyst.
This invention changes the dispersion and particle size distribution of the supported metal at a constant weight loading to achieve improved catalyst activity not obtainable by the prior art. The invention varies the particle size distribution of noble metal catalysts to increase the conversion for propane oxidation or nitric oxide reduction by 3-30%.